The invention relates generally to the field of threshold logic circuits and more particularly to the field of threshold logic circuits used for producing spark timing signals for an electronic ignitition system.
In electronic ignition systems for internal combustion engines it is generally desirable to produce a spark ignition signal at the spark plugs of the engine at fixed times with respect to the rotation of the drive shaft being driven by the engine. Typically, a magnetic pick-up coil is used to produce an AC signal having a frequency related to the rotational movement produced by the engine. This AC signal is then conventionally used to render a transistor, or other type of semiconductor component, conductive during certain portions of the AC signal. Thus the transistor is used to create a logic timing signal from the received AC signal produced by the magnetic pick-up. This timing signal is subsequently used to produce spark plug ignitions at predetermined angular positions of the drive shaft which is being rotated by the engine. One such electronic ignition system is illustrated in U.S. Pat. No. 3,933,141 Ser. No. 449,185 by Philip Gunderson which is entitled "Zero Crossing Circuit For Electronic Ignition System" and which is assigned to the same assignee as the present invention.
Prior art ignition circuits typically use a threshold level (switching level) which is effectively substantially different than the DC reference level upon which the AC magnetic pick-up signal is superimposed. This results in increasing the noise immunity of the prior art circuits by requiring larger AC signal magnitudes in order to produce changes in the output of the threshold semiconductor device. However, this also results in a non-symmetric switching of the threshold device by the AC signal which in turn results in the generation of inaccurate spark timing signals, especially at the extremely low engine speeds encountered during the starting up of the engine.
One prior art ignition system which does provide for symmetric, zero crossing, switching is illustrated in the previously mentioned patent. This prior art system does provide accurate spark timing signals throughout all modes of operation of the engine. However, this prior art system does have a problem when used in a conventional ignition system since it is especially susceptible to small magnitude noise pulses induced in the magnetic pick-up coil. These pulses may cause an undesired switching of the threshold device which may result in the creation of an unwanted spark. In the engine run mode of operation, this additional unwanted spark may cause a cylinder to misfire and therefore inhibit the efficient operation of the engine. However, a much more serious problem is caused when such an unwanted spark is created before the engine has been started. In this case a noise pulse may create an undesired spark ignition which may ignite the fuel mixture in a cylinder. This in turn can result in a substantial cranking of the engine. The creation of a spark in the aforementioned circumstances could therefore result in either an undesired starting up of the engine or an undesired substantial rotation of the engine. Either of these consequences could be disastrous if an auto mechanic happens to be working upon the engine at the time of the creation of the undesired spark ignition.
The basic problem with the previously mentioned ignition system is caused by the fact that the threshold level of the switching device is effectively set to the DC reference level of the Ac signal created by the magnetic pick-up. Therefore any slightly positive or negative AC pick-up signal may trigger the switching device. While this condition creates the advantage of having symmetrical switching which results in accurate timing pulses throughout the operation of the engine, this effective equality of threshold and reference levels also creates the probability of the engine misfiring during high speed engine running conditions and prior to the desired start-up of the engine.
Some prior art ignition systems have provided for an internal hysteresis in the turning on of the threshold device. This amounts to requiring a higher threshold level for turning on the switching device and a different and lower threshold level for turning off the threshold device. These systems, just like systems which provide for a high (non-zero) and constant threshold level for the switching device, destroy all symmetrical switching and therefore prevent the development of accurate timing ignition pulses, especially at very low (cranking or starting) engine speeds.